1. Field of the Invention
This invention relates to an electrophotographic photosensitive member, more particularly, to an electrophotographic photosensitive member having durability which is excellent in humidity resistance and cleaning property.
2. Description of the Prior Art
As photoconductive materials for electrophotographic photosensitive members, there have been known inorganic photoconductive materials such as selenium, cadmium sulfide, zinc oxide and the like, and organic photoconductive materials such as polyvinyl carbazole, oxadiazole, phthalocyanine, and the like. Organic photoconductive materials, while they are excellent in productivity as compared with inorganic photoconductive materials, are lower in sensitivity. Accordingly, as an effective sensitization method, it is also known to use a constitution of function separation type in which a charge generation layer and a charge transport layer are laminated.
As a matter of course, an electrophotographic photosensitive member is required to be provided with sensitivity, electrical characteristics, and further optical characteristics corresponding to the electrophotographic process to be applied. Particularly, in a photosensitive member which can be used repeatedly, its surface layer is subjected directly with electrical and mechanical outer forces such as corona charging, toner development, transfer onto paper, cleaning treatment, and the like, and therefore, resistance to such treatments is required. More specifically, resistance to lowering in sensitivity or lowering in potential, increase of residual potential due to deterioration by ozone generated during corona charging, and generation of abrasion or flaws at the surface by friction is demanded. On the other hand, humidity resistance of a photosensitive member is also another important property. Even if a photosensitive member is provided with excellent electrophotographic characteristics under low humidity, stable and clear images can be obtained with difficulty, provided that the surface potential of the photosensitive member is markedly lowered under high humidity. Also, in an electrophotographic process in which transfer of image is conducted, since the photosensitive member is usually employed repeatedly, humidity resistance will frequently be lowered because of charging deterioration of the photosensitive member. Such lowering in humidity resistance may be prevented to some extent by heating the photosensitive member with a heater to remove moisture, but such a measure will result in increased cost, because the heater must be constantly actuated.
Further, attachment of paper powder through contact with paper may be a cause for image flow under high humidity. This may be considered as follows. While paper powder absorbs moisture in the air on one hand, the nitrogen oxide formed by corona discharging will be dissolved in the moisture thus absorbed to form ions which will lower the electrical resistance of the paper powder, whereby the portion attached with paper powder can bear charges with difficulty. Also, since the paper powder is caused to flow in the rotational or moving direction of the photosensitive member during the cleaning treatment, the portion bearing difficulty the charges will also be caused to flow, whereby blank areas bearing no toner due to difficulty in bearing charges will be generated in "flowed form". This is the image flow, and of course no image will be formed at the blank area. Also, the residual toner due to filming or cleaning badness of toner will impair markedly the images obtained. Accordingly, it has been demanded to form a photo-sensitive member surface which will hardly be contaminated or readily removed of contaminants, if contaminated.
In the prior art, various methods have been proposed to overcome the drawbacks as described above. As one method, it has been known to add by dispersing fine powder of a fluorine type polymer such as Teflon, polyvinylidene fluoride, etc. into the surface layer of a photosensitive member or cover a photosensitive layer with a resin layer in which these fine powders are dispersed. However, although considerable improved effect can be seen with respect to contamination prevention, humidity resistance, etc., it is not practical because there is observed a phenomenon that the residual potential is increased as the photosensitive member is used repeatedly to cause generation of fog in the image.
Also, it has been attempted to lower the coefficient of friction of the photosensitive member surface or impart humidity resistance to the photosensitive member surface by addition of a fluorine type or silicone type surfactant or oligomer into the surface layer of a photosensitive member. Its effect can be recognized fully at the initial stage, but as the photosensitive member is repeatedly used and the surface layer is scraped off mechanically by cleaning, etc., the effect will become extemely reduced.
Further, an example of using a polymer of a vinyl compound having fluorine atom in the side chain as a finder for a photoconductor so as to continue the effect is also known as disclosed in Japanese Patent Application Laid-open No. 37046/1977. However, such a polymer has generally a side chain of fluorine-substituted alkyl group and is therefore poor in compatibility with an organic photoconductive material, and also insufficient in mechanical strength, and it is difficult to produce a photosensitive member excellent in durability with the use of such a polymer.